Engines may operate using a plurality of different fuels, which may be separately delivered, or delivered in varying ratios, depending on operating conditions. The different fuels may have differing efficiencies at a given operating condition. For example, an engine may use a first fuel (e.g., ethanol) and a second fuel (e.g., gasoline), each with different knock suppression abilities, to reduce engine knock limitations while improving overall fuel economy. As such, there may be several reasons why different fuels available to the engine may have different efficiencies at various engine running conditions. As one example, the available fuels may have different octane ratings which affects spark retard usage and engine efficiency at high loads (for example, when the different fuels are compressed natural gas versus gasoline, or E85 versus gasoline, or regular quality fuel versus premium quality fuel). As another example, different fuels may have different pumping work (for example, when the different fuels include a gaseous fuel versus a liquid fuel, or a port injected fuel versus a direct injected fuel). As still another example, different fuels may have different parasitic losses (such as when the fuels include a fuel delivered via high pressure direct injection versus a fuel delivered via low pressure port injection).
Engine control systems may select a fuel for injecting into cylinders from the multiple available fuels based on engine operating conditions, fuel availability, as well as fuel costs. One example approach is shown by Surnilla et al. in U.S. Pat. No. 7,703,435. Therein, fuel selection is based on fuel availability, engine temperature, and knock limits. Another example approach is shown by Williams et al. in US20140067540. Therein fuel selection is based on fuel costs in a geographical area of interest.
However the inventors herein have recognized potential issues with such approaches. As one example, there may be constraints and trade-offs associated with the fuel selection, such as reduced efficiency, torque, or power when a particular fuel is selected for cost reasons. Another issue is that frequent changes in operator pedal demand, as well frequent pedal demand overshoot may cause the engine load to move back and forth, leading to frequent switching between the fuels. Excessive fuel switches can degrade fuel economy due to losses incurred during transitions. In addition, the frequent switching can result in speed/load and air/fuel ratio disturbances. The issue may be exacerbated in a hybrid vehicle where the engine encounters multiple engine pull-ups and pull-downs (such as during frequent start/stop events).
The inventors herein have recognized that the operating cost of a hybrid powertrain having a multi-fuel engine may be reduced (e.g., minimized) by determining a minimum cost of vehicle operation at the most efficient speed/load for each fuel at the driver demanded power, while compensating with battery power, and additionally while smoothing torque transients using motor torque. In particular, battery power can be leveraged to reduce the frequency of fuel switching while also improving the cost of operating with a given fuel, without being hindered by associated constraints and trade-offs. In one example, fuel economy may be improved by a method for a hybrid vehicle system comprising: propelling the vehicle via an engine combusting a first fuel and a second fuel selected based on driver demand; and in response to a change in driver demand, adjusting relative usage of the first fuel and the second fuel based on each of the change in driver demand and a battery state of charge. For example, a controller may select between maintaining usage of the first fuel or transitioning to the second fuel. As a result, frequent fuel switching and associated losses can be reduced.
As an example, a hybrid vehicle system may be configured with a battery powered electric motor for propelling vehicle wheels via motor torque, as well as a bi-fuel engine wherein one of two fuels is used for propelling vehicle wheels via engine torque. The two fuels may have different octane ratings and may be delivered to the engine via distinct delivery systems. As one example, the two fuels may include a higher octane ethanol fuel that is delivered to an engine cylinder via direct injection and a lower octane gasoline fuel that is delivered to the engine cylinder via port injection. At any given driver demand, the controller may be configured to compare the fuel efficiency versus power for each available fuel, including a fuel the engine is currently operating on as well as an alternate available fuel. Upon retrieving a cost of each fuel (such as from the cloud), the efficiency may be divided by the cost to determine a “work per dollar” value for each fuel. The controller may then recalculate the efficiency of each fuel with a range of battery offsets. The battery offsets may be determined based on the state of charge of the system battery and may include a positive offset (wherein battery power via battery discharging is used to boost engine output) as well as a negative offset (wherein battery power via battery charging is used to adjust engine output). Assuming an average cycle efficiency for battery power generated from the engine, the controller may calculate the “battery work per dollar” values for each fuel. The controller may then select whether to continue using the current fuel (with or without battery offset) or transition to using the other fuel (with or without battery offset) by comparing the costs. Specifically, if a higher than threshold improvement in efficiency and cost is achieved by transitioning to the other fuel, the transition may be performed, else usage of the current fuel may be maintained. Any transients incurred during the transition may be smoothened using motor torque. Also following the fuel selection, the controller may use motor torque adjustments to operate the engine in a narrow speed-load operating range where efficiency of the selected fuel is optimized, while maintaining a given power level of the vehicle.
In this way, fuel economy losses in a vehicle system can be reduced. One of the technical effects of using battery power to extend operation of a multi-fuel engine with a given fuel in a hybrid vehicle is that losses associated with frequent fuel switching are reduced. In particular, battery power can be used to keep operating the engine on a current fuel at a more efficient power. While operating the engine with the more efficient and cost-effective fuel, battery power can be used up to a threshold to make up any difference in output, the threshold based on an associated cost penalty. The technical effect of using battery power to meet driver demand while maintaining a cost-effective and efficient fuel usage in a multi-fuel engine during selected engine operating conditions is that fuel switching can be reduced. In addition, engine operation in a more efficient and cost-effective fuel regime can be extended despite changes in driver or wheel torque demand.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.